I assume you're aware that there are (off the top of my head) three Linux distros already ported to the RPi, and available as sdcard image downloads from the RPi website.
However, I believe one of these would have to be adapted to use a real time kernel before it would be suitable to use with EMC2. I have not looked at what RPi peripherals these existing ports support. Let's assume that some additional simple drivers might be needed too.
Still, it doesn't seem like a huge amount of work.